Advanced CAD Modeling by Nikola Vukašinović & Jože Duhovnik

Author:Nikola Vukašinović & Jože Duhovnik
Language: eng
Format: epub
ISBN: 9783030023997
Publisher: Springer International Publishing

All technologies for the mass production of identical products share a common feature—they are based on the use of forming tools, custom-made for each product, as making a specific tool becomes economically justified only with large quantities of a single product. The most important technologies for the manufacture of large quantities of products include closed-die forging and the stamping of metal products, die casting and the injection moulding of hot metals, injection moulding and the blowing of polymer (plastic) products, and the sintering of ceramic products. Another common issue with all these technologies is the requirement for the fastest possible duty cycle of the tool and material mass flow through the tool. What makes it interesting is that despite very different technologies and materials, there are very similar design recommendations regarding the geometry of products in order to make the manufacturing cycle smooth and of acceptable quality. Another reason lies in the requirement to remove the products from the tool immediately after the technological process, and that deformation and shrinkage of the product, coming from the technological process, should be predictable and even. Both requirements can be satisfied by taking into account the basic physical and geometric laws (e.g., elasticity or thermal expansion of material), which otherwise applies to any manufacturing technology. This makes the guidelines for making tools similar: such draft angles should be applied that a free flow of material is provided and the most uniform possible distribution (thickness) of material across the entire product is provided, in order to achieve a steady cool-down and shrinkage of the product.

The basic requirement for casting and injecting into moulds is for the geometry of the product that is being moulded to allow quick and easy removal of the product from the tool. It means that the product’s walls should have no negative angles, relative to the tool’s removal. Even more important, angles should be positive enough to allow the effortless removal of the product after it has solidified and cooled down. The minimum required angle depends on the length of removal, the casting technology and the material, the wall thickness, the product geometry and the cool-down shrinkage. Recommended rough values of the draft (mould) angles for the various casting and injection technologies are in Table 5.1. The table represents only the basic design guidelines for making castings; for a perfect product in all its details it is recommended to consult a technician and perform the appropriate injection and casting simulations. It is the complex interactions between many parameters that determine the final decision on the required draft angles. They include wall length, wall thickness, prescribed surface texture, properties of the casting material and tool, etc. As a product usually shrinks in the direction of its centre of mass, its external walls shrink away from the mould walls, and the internal walls generally shrink towards the mould walls. It makes the draft angles of the product’s interior walls a vital issue in order to prevent a casting from shrinking onto the mould.

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